Gas turbine engine components, such as fan cases and bleed valves are typically constructed of aluminum and magnesium alloys. Aluminum alloys such as AA 6061 and AA 2024 are soft and suffer damage via general wear, especially when two components are in sliding contact with each other. One method of mitigating wear of aluminum alloys is an anodic conversion of the alloy to produce a hard oxide layer on the exposed surface via processes such as AMS 2469. Hard anodic coatings generally provide superior wear resistance then other methods of restoring aluminum alloy components dimensionally.
However, hard anodizing processes have drawbacks. Hard anodizing processes consume part of the surface of the components. The hard anodizing process adds thickness to the surface, but the interface between anodized surface and parent alloy moves in to the parent alloy. A rework or repair of the anodic layer requires that all prior layers be removed and therefore removal of an anodic layer also involves removal of some of the original material of the component, thereby altering the original dimensions of the component. As a result, the number of times a hard anodized layer may be applied in the rework and repair of the component is limited. Often, plating or welding of the component surface to restore the original dimensions is required, which is costly and time-consuming.
As an improvement to hard anodic coatings like AMS 2468, polytetrafluoroethylene (PTFE) has been added to provide a different coating, such as AMS 2482, which provides improved performance for sliding contact surfaces.
In a gas turbine engine, bleed valves are but one example of components that include sliding contact surfaces that are prone to wear. Another example can be found in the slots formed at the aft and of a fan exit case that are used to couple the fan exit case to the cowl doors that form part of the nacelle that encloses the engine.
With respect to bleed valves, compressors of gas turbine engines are designed to operate at one optimum speed. Each rotating compressor blade and each stationary stator vane are made to operate most efficiently at a certain airflow and pressure. If the engine operates at any speed less than, or greater than this “design” speed, the efficiency of the compressor decreases. At very low speeds, such as starting, and idling, a compressor is operating outside of its efficient running zone. At higher compressor compression ratios, the engine becomes more efficient. Therefore, at a low speed operation of a gas turbine engine, typically encountered when starting or idling, the compressor discharge pressure can literally turn around and exit out the front of the engine, or “stall”. A stall can be severe, and can lead to the engine stopping or being damaged. At these low speeds, the front stages of the compressor pull in more air than the higher pressure stages can handle. As a result, the high flow rate “chokes” in the higher stages, pressure builds up in the middle of the compressor, and the slow moving front stages cannot hold the pressurized air back any longer. The air then simply reverses, and blows out the inlet duct.
To remedy this problem, engine designers add one or more bleed valves on the compressor case. The bleed valves allow this extra air being brought into the engine by the front stages to be blown into a bypass flow path, thus keeping the airflow in these stages high, the air pressure low, thereby enabling the higher stages of the compressor to handle the reduced airflow and lower pressures efficiently. Once the engine speeds up, the bleed valves will close to keep the compressor operating within its efficient zone, until reaching full speed, where all bleed valves are closed, and the compressor reaches or approaches its peak efficiency.
Typical bleed valves include a valve element selectively movable to an open position where it provides communication between bypass flow path and the primary flow path, such that some of the air from the primary flow path can be directed to the bypass flow path. The valve element and the various sliding contact surfaces it engages are prone to wear. Another example of gas turbine engine services subject to sliding wear are the slots that form part of the connection between a fan exit case and the cowl doors that form part of the nacelle.
To combat the effects of wear and prolong the life of bleed valves or other sliding contact where surfaces, special coatings are applied. One such coating, AMS 2482 is a hard aluminum oxide coating, impregnated or co-deposited with polytetrafluoroethylene (PTFE). However, when the layer or coating of AMS 2482 is worn away, the AMS 2482 typically cannot be replaced without plating or welding the surface of the part to restore the original dimensions, which is time consuming and costly.
Thus, improved coatings and improved sliding contact surfaces are needed to reduce maintenance and parts costs.